Life through Time and Space

By Wallace Arthur and Stephen Arthur

All humans share three origins: the beginning of our individual lives, the appearance of life on Earth, and the formation of our planetary home. Life through Time and Space brings together the latest discoveries in both biology and astronomy to examine our deepest questions about where we came from, where we are going, and whether we are alone in the cosmos.

A distinctive voice in the growing field of astrobiology, Wallace Arthur combines embryological, evolutionary, and cosmological perspectives to tell the story of life on Earth and its potential to exist elsewhere in the universe. He guides us on a journey through the myriad events that started with the big bang and led to the universe we inhabit today. Along the way, readers learn about the evolution of life from a primordial soup of organic molecules to complex plants and animals, about Earth’s geological transformation from barren rock to diverse ecosystems, and about human development from embryo to infant to adult. Arthur looks closely at the history of mass extinctions and the prospects for humanity’s future on our precious planet.

Do intelligent aliens exist on a distant planet in the Milky Way, sharing the three origins that characterize all life on Earth? In addressing this question, Life through Time and Space tackles the many riddles of our place and fate in the universe that have intrigued human beings since they first gazed in wonder at the nighttime sky.

• Language: English
• Publisher: Harvard University Press
• Release date: Aug 7, 2017
• ISBN: 9780674982277

Wallace Arthur

Wallace Arthur is a professor of zoology at the National University of Ireland, Galway. The author of books including Creatures of Accident, he also serves as European editor of the journal Evolution and Development.

Book preview

Arthur

Preface

I’ve spent most of my life as a biologist; and in the last few years I’ve spent a lot of time studying astronomy. This book is a celebration of these sciences and of the growing relationship between them—a celebratory story told in plain language, not technical jargon. It’s about our origins, our fates, our place in the universe, and the likelihood of intelligent alien life, looked at from both biological and astronomical points of view. I dedicate the book to all those who help to promote understanding rather than indoctrination, and in particular that great truth-seeker Thomas Henry Huxley. Understanding is one of the noblest goals of humanity, and one of the keys to our survival and progress.

The book is written as a series of seven triplets of chapters. Within each triplet, the first chapter is predominantly astronomical or astrobiological in flavor, the second is evolutionary, and the third is embryological. However, each chapter has arms that reach out into one or both of the other two domains. The connections between triplets, and between chapters within triplets, might at first seem cryptic, but they have a curious logic. I can best illustrate the nature of these connections by using the first triplet as an example.

In Chapter 1 we consider hypothetical (but maybe also real) inhabitants of the Andromeda galaxy, an object that can be seen in the nighttime sky by anyone with reasonably good eyesight. We use these extraterrestrial creatures as an entry point to the possibility of alien life in general. We imagine them looking toward the Earth with telescopes so advanced that they can actually see not just our planet but individual people wandering over its surface.

In Chapter 2 we meet some of the people they see. But these would not be us. Rather, since light from Andromeda takes 2.5 million years to reach us, and the same span of time to travel in the other direction, they would see protopeople of the distant past, belonging to an early species of Homo, characterized by a brain that’s about half the size of our own.

In Chapter 3 we acknowledge that characterizing a species of human or protohuman as having a brain of a specific size is an oversimplification. As individuals, our brains are at first nonexistent, then small, then large. Early human embryos lack not only brains but any nerve cells at all. So, we contemplate the form of these early embryos, and the question of how they go about producing the beginnings of their nervous systems.

The other connections between chapters work in similar ways, so I won’t waste words elucidating them further here. Their individual details will reveal themselves soon enough. The overall pattern of their linkage is designed to take you on a fascinating journey through embryological and evolutionary time, terrestrial and interstellar space.

I

FROM STARS TO EMBRYOS

Chapter One

Galaxy Gazing

The Big W in the Sky

On a cloudless night, step outside and gaze up at the sky. What do you see? The short answer to this question is suns—or stars, which is a different name for the same thing. There are lots of them; just how many you see depends on where you live. If you’re in a big city, you’ll perhaps see only tens of them. If you’re deep in the countryside, where the level of light pollution is low, you’ll be able to see hundreds or, if you use binoculars, thousands. Not quite all of the bright objects you see in the sky will be stars. The exceptions will probably be a couple of planets, the lights of a few planes, and perhaps an orbiting satellite. What I want to direct your attention to, though, is a strange fuzzy blob which is none of these things. It’s a galaxy, and one that almost certainly harbors intelligent life. When you look up at it, there is someone, or something, looking back at you.

The object I’m thinking of is the great galaxy of Andromeda. To find it, you can use a group of five bright stars that form the shape of a W. This is the constellation Cassiopeia, named after a vain queen in Greek mythology. It’s one of the most conspicuous in the nighttime sky, if you live in the Northern Hemisphere. It can be seen year-round, even from quite light-polluted localities. Once you know that there’s a big W in the sky, it’s quite easy to find. Of course, a W is really two Vs stuck together. To locate the Andromeda galaxy, you use the right-hand V of this particular W as an arrowhead.

Here’s what you do. Look at how deep the V is. Then project the angle of your view down about three times that depth, in the direction of the arrowhead and very slightly skewed to the right. There you will find the Andromeda galaxy.

How easy is this object to see with the naked eye? This depends on just three things. The first is your eyesight. If you have good sight, you should see it. If, like me, you have only average sight, you might see it and you might not—but with a pair of binoculars you’ll be fine. The second is the level of light pollution. If you live in a large city, you might have to go out into the surrounding countryside to be able to see the galaxy. The third is the time of year. Although the big W is visible in all seasons, when you project the direction of your gaze the appropriate distance in the direction of the arrowhead, there is a time of year when this will take you to a point close to, or below, the horizon. For many of us, this will be just a short period in the spring; the exact duration depends on where you live.

Meet the Andromedans

Once you see the galaxy, consider the following. It looks like a little fuzzy blob, but it consists of about 500 billion suns (or stars). Orbiting most of those suns are planets, some of them much like our own Earth. On many of those planets, there are life-forms. This may at first seem like an overly strong statement: why many rather than a few? And why no probably? We’ll soon see the answers to these questions; for the moment, please trust me that the sums work out in such a way that the likelihood of there being no life at all in the Andromeda galaxy is negligible.

Let’s imagine two humanoid Andromedan scientists looking exactly in our direction, with a hypothetical telescope so powerful that they can see not just our planet but also individual humans walking across its surface. Even if such a device were possible, when these Andromedans look directly at us they do not see us. Why not?

Space and Time

To answer this question, we need to think a bit about space and time. The Andromeda galaxy is very close to us, as galaxies go. Admittedly it’s many trillions of kilometers, or miles, away, but it’s within our local group of galaxies (I love that phrase)—the ones that are really, really close to us, compared with all the others. Since neither kilometers nor miles work well for us when it comes to comprehending the vast distances of intergalactic space, we use other units, one of which is the light-year. We’ll have to get thoroughly on top of this unit to understand exactly how far away Andromeda and other galaxies are from us. It has to become something that’s as familiar to us as a meter or an inch.

The first thing to be clear about is that, despite its name, a light-year is a measure of distance, not time. You may already know this, in which case you’ll probably also know that it’s the distance light travels through space in a year. But how far is that? We can easily work it out. I was taught, as a child, that light travels at 186,000 miles per second. And so it does. But if you were taught using metric rather than U.S. customary or imperial units, then you’ll have been told that light travels at 300,000 kilometers per second—which is the same thing, though a suspiciously round figure. If we know how far light travels in a second, it’s easy to calculate how far it will travel in a longer period of time, like a year. To save you doing the sums, here’s the answer: very roughly 10 trillion kilometers, or 6 trillion miles (these figures are not suspiciously neat; I’ve just rounded them to the nearest trillion).

Using this astro-friendly unit, how far away is the Andromeda galaxy? The answer: approximately 2.5 million light-years. Close enough to be local in astronomic terms (many galaxies are billions of light-years from us), but rather a long way from us in any other terms.

Now let’s move from distances in space to distances in time. Actually, this is very straightforward, given that we’re starting with the distance unit that we call a light-year. The time that light takes to reach us from Andromeda is, by definition, 2.5 million years. So when we look at the galaxy from Earth, we see it as it was 2.5 million years ago, when the light we’re seeing right now was originally emitted from the galaxy and began traveling toward us.

Watching Our Ancestors

This looking back in time works both ways. So if those Andromedan scientists were looking in our precise direction last night, they won’t have seen us. But they may have seen some protohumans, perhaps belonging to the species Homo habilis (literally, handy man; more information on these creatures will follow shortly).

I’m quite convinced that my hypothetical Andromedan scientists are real. Here’s why. Observations made over the last couple of decades on stars / suns within our own galaxy—the Milky Way—show that many suns, not just our own, have planets. Not only that, but suns with multiple rather than single planets are common, and may well be the norm. Solar systems with one, two, three, four, five, six, seven, and eight planets are all known—but with the proviso that in each case the figure I quote is a minimum because other planets in the appropriate system may yet remain undiscovered. It seems likely that systems with more than our own eight planets exist too, and will be found soon.

Planets of Life

So there are lots of planets. But how many are Earth-like? The best guess at the time of writing is about 1 in 200, though this figure may well have changed a bit by the time you’re reading this chapter, given that a typical book has a gestation period of about a year and the current rate of planet discovery is remarkably high.

This figure of 1 in 200 is reached as follows. We now know of about 4,000 confirmed exoplanets—the name given to planets orbiting suns other than our own. Of these, about 20 are Earth-like, though of course that leaves open the question of exactly how Earth-like. If this is a fair sample of our galaxy overall (the Milky Way is a bit smaller than the Andromeda galaxy), then, when we know more, the numbers 20 and 4,000 will simply be scaled up and the fraction of Earth-like planets will remain about the same. Assuming that the Milky Way and Andromeda are broadly similar in their composition, which seems likely, the fraction of planets that are Earth-like there will be approximately the same as it is here.

The route from the Andromeda galaxy to the likelihood of Andromedan life-forms works something like this. We’ll guesstimate the number of planets in Andromeda as being the same as the number of stars—about 500 billion. That’s probably an underestimate, but no matter; in fact, it’s sensible to err on the cautious side when trying to estimate the likelihood of life. Now we can guesstimate the number of Earth-like planets as being 1 / 200 of this huge number, which works out to 2.5 billion. We’ll be pessimistic about the fraction of these that embark on an evolutionary process that produces life—say, just 1 in 100, which is probably another underestimate.

This gives us 25 million planets with life. On what fraction of these has evolution produced intelligent life? Let’s go with our 1 in 100 fraction again, so we’re now down to 250,000 planets. So our best guess is that intelligent life exists on about 250,000 planets within the Andromeda galaxy. Well, actually, no, because there’s another factor to be taken into account: how long does intelligent life tend to last once it has evolved? We can’t yet answer this question. Nevertheless, the idea that intelligent life is almost always doomed to an early grave would have to be taken to extreme lengths to reduce the guesstimated number of Andromedan civilizations from a quarter of a million to zero.

Anyhow, that’s as far as I want to take the guesstimates. The American astronomer Frank Drake, one of the pioneers of SETI (search for extraterrestrial intelligence), went a bit further. He produced an equation for, effectively, the likelihood of intelligent life—not in Andromeda but in our own Milky Way galaxy. However, I don’t think we need equations to grasp the general picture, and such pictures are what this book is all about.

Relatedness across the Sky

Now that we’ve established a high likelihood of the existence of intelligent Andromedans, let’s ask how they are related to us. In one sense, the answer is not at all. But let’s expand on that a little.

To think about our relatedness to any creature here on Earth, it’s helpful to use the letter Y. Time runs upward through the letter, and two processes that separate from each other over time do so at the point where the stem of the letter splits. Take the relationship between humans and chimps. In this case the point of separation was about 7 million years ago. If, instead, we inquire about the relatedness of humans and dogs, the point of separation, or divergence, was closer to 70 million years ago than 7 million.

By choosing ever more distantly related creatures, we can reach ever earlier divergence times. The point at which the lineage leading to the genus Homo (humans) diverged from the lineage leading to Spongia (a genus of sponges, unsurprisingly) was probably around 700 million years ago—up by another order of magnitude. But there is no creature to which our relatedness is such that our lineages diverged from each other another order of magnitude longer ago than this, because 7 billion years ago the Earth had not yet been born.

Now, back to the Andromedans. Is it futile to seek a Y that connects them and us? Is a divergence time for their lineage and ours a meaningful concept? Probably yes and no, respectively. But let’s not dismiss the idea of relatedness without digging a little deeper. Some people, including the Swedish chemist Svante Arrhenius and the British astronomer Sir Fred Hoyle, have speculated about the possibility of life on Earth having originated from the arrival here of a spore or other hardy life-unit from space—the idea being that life originated somewhere else (where?) and that our lineage is ultimately related to lineages that may still exist on some other planet. If interstellar migration of life-forms is possible (as a biologist, I doubt it), then there could indeed be a Y that connects the human lineage here with a humanoid lineage elsewhere.

However unlikely the interstellar migration of life-forms, the likelihood of intergalactic migration is even lower. What kind of spore could survive more than 2 million years of space travel at the speed of light, or even longer at a lower speed? None that we know of, for sure. So in the end it’s likely that no living stem connects us with Andromedan life-forms. But what about a non-living stem? In the conventional evolutionary biology of Earth-bound creatures, such a possibility is (quite rightly) not considered. But let’s think outside the proverbial box.

What I’m getting at here is the question of whether there was some ancient material—perhaps gas or dust—from which both the Milky Way and Andromeda, and hence all their constituent creatures, formed. Strangely, there is not yet a consensus about this issue: we remain somewhat in the dark about how galaxies were born. We’ll come back to that particular type of origin later. For now, let’s just say that if there was a common cloud from which we and the Andromedans came, it existed many billions of years ago.

Life on the Wing?

You may have noticed something interesting that snuck in untrumpeted in the previous paragraph. This was the idea of all the constituent creatures of the Milky Way—the implication being, of course, that there is alien life much closer to us than Andromeda. If that’s true (it probably is, but we don’t know for sure), why have I started out by asking you to consider the possibility of life so far away? The answer is that I want you to be able to look at one specific thing in the sky where we’re pretty sure life exists. When that thing is a fuzzy blob that contains billions of suns, we can indeed be pretty sure. However, when it’s a single star, the chances of there being life on one of its orbiting planets are actually quite low.

You’ll recall that in the recipe for finding the Andromeda galaxy the first step was to locate the big W in the sky that we call Cassiopeia. The second was to consider the W as being made up of two Vs, the right-hand one of which we used as an arrowhead. The three stars of that arrowhead, from the right-hand edge inward, are Caph, Schedar (the spelling is somewhat variable), and Navi. Perhaps we didn’t need to use these as an arrowhead to point to something else—perhaps these suns / stars may themselves have orbiting planets with life. So far, there is no evidence to suggest that they do. However, if we journey to another constellation in the northern sky, Cygnus the swan, there is a star called Kepler 186 that has a planet (186f) that is rather Earth-like and may well host life. This solar system is in the area of the swan’s right wing. And there are many others in the same general direction.

The Excitement of Science

Now here’s an important issue for what we call popular science. The main aim of this difficult endeavor is to spread the findings, and indeed the excitement, of science, with a minimum of turgid detail. To do science, details are crucial. But to learn about science’s big picture of things, they’re not. Or, to be a bit more accurate, they can be minimized. And that’s what I’ve been trying to do so far in this book, and will continue to do throughout, following in the tradition of others who have written in this genre.

But wait a minute: have I succeeded up to this point? Maybe not. Here’s a list of the astronomical terms I’ve mentioned so far: Cassiopeia, Andromeda, Milky Way, galaxy, light-year, exoplanet, Caph, Schedar, Navi, Cygnus, Kepler 186 (sun), Kepler 186f (planet).

That’s already a dozen potentially new names. If you’re an astronomer, probably none of them will actually be new. But for most people some will be new, and for some people most will be new. How can anyone commit to memory a list of new names without getting bored and losing sight of the big picture and the excitement of scientific discovery? It’s vital to provide an answer to this question, for otherwise the mysteries of the universe will be eclipsed by detail, jargon, names. Any author guilty of achieving that appalling eclipse (and there are many) should be ashamed. I will try very hard not to fall into the jargon trap, though 12 potentially new names in fewer than that number of pages does not seem an auspicious start.

But there’s a solution to this problem: replacing many individual names with a single framework on which to hang them. For the names we’ve encountered so far, here’s such a framework.

Close, Middling, and Far

There are three domains of space: close, middling, and far. Close contains only our own solar system—the Sun, the Earth, the other familiar planets (Mercury, Venus, and so on), the Moon, and a motley collection of other things (asteroids, dwarf planets, comets). From the perspective of another star, such as Caph, this whole collection of stuff can be thought of as just a pinpoint in space. Indeed, that’s exactly what it would look like from Caph—it would appear as a fairly ordinary star that, if magically zoomed in upon, would reveal all this extraordinary detail including, ultimately, humans.

Middling contains all the other stars of our Milky Way galaxy. Take the arrowhead of Caph, Schedar, and Navi, for example. Although the arrowhead and the W of which it is a part seem like flat entities in the night sky (as do constellations in general, because we can’t really detect the third dimension of celestial depth), they’re very far from flat indeed. Caph is about 50 light-years away. Schedar, at about 200 light-years, is roughly four times as distant. And Navi is approximately three times farther again, at about 600 light-years. However, in one important sense, these distances are all the same—that is to say, they’re all middling.

To see how middling differs from close, consider this. The full span of our solar system (comets and other oddballs aside) from the Sun to the average orbital distance of the farthest-out planet, Neptune, is only a tiny fraction of a light-year—less than a thousandth, in fact. We don’t even use the light-year as a unit of measurement at this spatial scale. But the closest star to us—in other words, the closest sun apart from ours—is more than four light-years away. So the closest star is more than 4,000 times as far away from us as is the farthest planet of our solar system. Truly, the close and the middling are different realms of